Technical Aspects of Mammography Image Acquisition

Mammograms are obtained on specially designed, dedicated x-ray machines using either x-ray film and paired fluorescent screens or digital detectors to capture the image. All mammography units are comprised of a rotating anode x-ray tube with matched filtration for soft-tissue imaging, a breast compression plate, a moving grid, an x-ray image receptor, and an automatic exposure control (AEC) device that can be placed under or detect the densest portion of the breast, all mounted on a rotating C-arm (Fig. 1-1). A technologist compresses the patient’s breast between the image receptor and compression plate for a few seconds during each exposure. Breast compression is important because it spreads normal fibroglandular tissues so that cancers, which have similar attenuation properties to fibroglandular tissues, can be better seen. Breast compression also decreases breast thickness, thereby decreasing exposure time, radiation dose to the breast, and the potential for image blurring as a result of patient motion and unsharpness.

Figure 1-1 Components of an x-ray mammography unit.

Women worry about breast pain from breast compression and about the radiation dose from mammography. Breast pain during compression varies among individuals and can be decreased by obtaining the mammogram 7 to 10 days after the onset of menses, when the breasts are least painful. Breast pain can also be minimized by taking oral analgesics such as acetaminophen before the mammogram or by using appropriately designed foam pads that cushion the breast without adversely affecting image quality or increasing breast dose.

Current mammography delivers a low dose of radiation to the breast. The most radiosensitive tissues in the breast are the fibroglandular tissues. The best measure of breast dose is mean glandular dose, or the average absorbed dose of ionizing radiation to the radiosensitive fibroglandular tissues. The mean glandular dose received by the average woman is approximately 2 mGy (0.2 rad) per exposure, or 4 mGy (0.4 rad) for a typical two-view examination. Radiation doses to thinner compressed breasts are substantially lower than doses to thicker breasts.

The main radiation risk from mammography is the possible induction of breast cancer 5 to 30 years after exposure. The estimated risk of inducing breast cancer is linearly proportional to the radiation dose and inversely related to age at exposure. The lifetime risk of inducing a fatal breast cancer as a result of two-view mammography in women age 45 at exposure is estimated to be about 1 in 100,000. For a woman age 65 at exposure, the risk is less than 0.3 in 100,000. The benefit of screening mammography is the detection of breast cancer before it is clinically apparent. The likelihood of an invasive or in-situ cancer being present in a woman at age 45 is about 1 in 500. The likelihood that the cancer would be fatal in the absence of mammography screening is about 1 in 4, and the likelihood that screening mammography will convey a mortality benefit is 15% (RCT estimates) to 40% (service screening estimates). Hence, the likelihood of screening mammography saving a woman’s life at this age is about 1 in 5000 to 1 in 13,000, yielding a benefit-to-risk ratio of 8 : 1 to 20 : 1. For a woman age 65 at screening, the likelihood of a mortality benefit from mammography is about 1 in 2000 to 1 in 4000 (assuming a 25% to 50% mortality benefit), yielding a benefit-to-risk ratio of approximately 90 : 1 to 180 : 1. Screening mammography is only effective when regular periodic examination is performed.

The generator for a mammography system provides power to the x-ray tube. The peak kilovoltage (kVp) of mammography systems is lower than that of conventional x-ray systems, because it is desirable to use softer x-ray beams to increase both soft-tissue contrast and the absorption of x-rays in the cassette phosphor (absorption efficiency), especially for screen-film mammography (SFM). Typical kVp values for mammography are 24 to 32 kVp for molybdenum targets, 26 to 35 kVp for rhodium or tungsten targets. A key feature of mammography generators is the electron beam current (milliampere [mA]) rating of the system. The higher the mA rating, the shorter the exposure time for total tube output (milliampere second [mAs]). A compressed breast of average thickness (5 cm) requires about 150 mAs at 26 kVp to achieve proper film densities in SFM. If the tube rating is 100 mA (typical of the larger focal spots used for nonmagnification mammography), the exposure time would be 1.5 seconds. A higher-output system with 150-mA output would cut the exposure time to 1 second for the same compressed breast thickness and kVp setting. Because of the wide range of breast thicknesses, exposures require mAs values ranging from 10 to several hundred mAs. Specifications for generators are listed in Box 1-3.

The most commonly used anode/filter combination is Mo/Mo: a molybdenum (Mo) anode (or target) and a Mo filter (25–30 microns thick), especially for thinner compressed breasts (<5 cm thick). Most current manufacturers also offer a rhodium (Rh) filter, to be used with the Mo target (Rh/Mo), to produce a slightly more penetrating (harder) x-ray beam for use with thicker breasts. Some manufacturers offer other target materials, such as Rh/Rh: a rhodium target paired with a rhodium filter, or tungsten (W), which is paired with a rhodium filter (W/Rh) or aluminum (Al) filter (W/Al). These anode/filter combinations are designed for thicker (>5 cm) and denser breasts. Typically, higher kVp settings are also used with these alternative target/filter combinations to result in a harder x-ray beam for thicker breasts, because fewer x-rays are attenuated with a harder x-ray beam (Box 1-4). One of the best parameters to measure the hardness or penetrating capability of an x-ray beam is the half-value layer (HVL), which represents the thickness of aluminum that reduces the exposure by one half. The harder the x-ray beam, the higher the HVL. The typical HVL for mammography is 0.3 to 0.5 mm of Al. The Food and Drug Administration (FDA) requires that the HVL for mammography cannot be less than kVp/100 ± 0.03 (in mm of Al), so that the x-ray beam is not too soft. For example, at 28 kVp, the HVL cannot be less than 0.31 mm of Al. There is also an upper limit on the half-value layer that depends on the target-filter combination. For the upper limit of Mo/Mo, the HVL must be less than kVp/100 + 0.12 (in mm of Al); so for 28 kVp, the HVL must be less than 0.4 mm of Al.

The size of the larger mammography focal spot used for standard, contact (i.e., nonmagnification) mammography is typically 0.3 mm. Magnification mammography requires a smaller focal spot, about 0.1 mm, to reduce penumbra (geometric blurring of structures in the breast produced due to the breast being closer to the x-ray source and farther from the image receptor to produce greater “geometric” magnification). The effect of focal spot size on resolution in the breast is tested by placing a line pair pattern in the location of the breast, at a specific distance (4.5 cm) from the breast support surface. For SFM, the larger mammography focal spot used for standard, contact mammography should produce an image that resolves at least 11 line pairs/millimeter (11 lp/mm) when the lines of the test pattern run in the direction perpendicular to the length of the focal spot (this measures the blurring effect of the length of the focal spot) and at least 13 lp/mm when the lines run parallel to the focal spot (measuring the blurring effect of the width of the focal spot). Thus, although the SFM image receptor can resolve 18 to 21 lp/mm, the geometry of the breast in contact mammography and the finite-sized larger focal spot reduce the limiting spatial resolution of the system to 11 to 15 lp/mm in the breast. The limiting spatial resolution of digital mammography systems is less (5–10 lp/mm), due to pixelization of the image by the digital image receptor. In digital, a “line” is 1 pixel width, and a line pair is 2 pixels. For example, for a digital detector with 100 micron (0.1 mm) pixel size or pitch (the center-to-center distance between adjacent pixels), a line pair consists of 2 pixels or 200 microns (0.2 mm). Therefore, one can fit 5 line pairs (at 0.2 mm each) into a 1 mm length, or the detector has a limiting spatial resolution of 5 lp/mm. By similar reasoning, a digital detector with 50 micron pixels has a limiting spatial resolution of 10 lp/mm.

The x-ray tube and image receptor are mounted on opposite ends of a rotating C-arm to obtain mammograms in almost any projection. The source-to-image receptor distance (SID) for mammography units must be at least 55 cm for contact mammography. Most systems have SIDs of 65 to 70 cm.

Geometric magnification is achieved by moving the breast farther from the image receptor (closer to the x-ray tube) and switching to a small focal spot (Fig. 1-2). Placing the breast halfway between the focal spot and the image receptor (as in Fig. 1-2B) would magnify the breast by a factor of 2.0 from its actual size to the image size because of the divergence of the x-ray beam. The MQSA requires that mammography units with magnification capabilities must provide at least one fixed magnification factor of between 1.4 and 2.0 (Table 1-1). Geometric magnification makes small, high-contrast structures such as microcalcifications more visible by making them larger relative to the noise pattern in the image (increasing their signal-to-noise ratio [SNR]). Optically or electronically magnifying a contact image, as would be done with a magnifier on SFM or using a zoom factor greater than 1 on a digital mammogram, does not increase the SNR of the object relative to the background, because both are increased in size equally. To avoid excess blurring of the image with geometric magnification, it is important to use a sufficiently small focal spot (usually 0.1 mm nominal size) and not too large a magnification factor (2.0 or less). When the small focal spot is selected for geometric magnification, the x-ray tube output is decreased by a factor of 3 to 4 (to 25–40 mA) compared to that from a large focal spot (80–150 mA). This can extend imaging times for magnification mammography, even though the grid is removed in magnification mammography. The air gap between the breast and image receptor provides adequate scatter rejection in magnification mammography without the use of an antiscatter grid.

Figure 1-2 Magnification mammography improves resolution. Nonmagnified, or contact, mammography (A) and geometrically magnified mammography (B). Using a small or microfocal (0.1 mm) focal spot with the configuration shown in part B, higher spatial resolution can be obtained in the breast compared to part A, where a larger (0.3 mm) focal spot is used. C, Craniocaudal mammogram shows a possible benign mass in the inner breast. D, Microfocal magnification shows irregular borders not seen on the standard view.

Table 1-1 Mammography Focal Spot Sizes and Source-to-Image Distances

The Mammography Quality Standards Act requires magnification factors between 1.4 and 2.0 for systems designed to perform magnification mammography.

Collimators control the size and shape of the x-ray beam to decrease patient exposure to tissues beyond the compressed breast and image receptor. In mammography, the x-ray beam is collimated to a rectangular field to match the image receptor rather than the breast contour, because x-rays striking the image receptor outside the breast do not contribute to breast dose. By federal regulation, the x-ray field cannot extend beyond the chest wall of the image receptor by more than 2% of the SID. So, for a 60-cm SID unit, the x-ray beam can extend beyond the chest wall edge of the image receptor by no more than 1.2 cm.

The compression plate and image receptor assembly hold the breast motionless during the exposure, decreasing the breast thickness and providing tight compression, better separating fibroglandular elements in the breast (Fig. 1-3). The compression plate has a posterior lip that is more than 3 cm high and usually is oriented at 90 degrees to the plane of the compression plate at the chest wall. This lip keeps chest wall structures from superimposing and obscuring posterior breast tissue in the image. The compression plate must be able to compress the breast for up to 1 minute with a compression force of 25 to 45 pounds. The compression plate can be advanced by a foot-controlled motorized device and adjusted more finely with hand controls.

Figure 1-3 Schematic of a compression paddle and image receptor showing the components of the cassette holder, the compression plate, and the breast. The film emulsion faces the screen. AEC, automatic exposure control.

(Adapted from Farria DM, Kimme-Smith C, Bassett LW: Equipment, processing, and image receptor. In Bassett LW, editor: Diagnosis of diseases of the breast, Philadelphia, WB Saunders, 1997, pp 32 and 34.)

For positioning, the technologist tailors the mammogram to the individual woman’s body habitus to get the best image. The breast is relatively fixed in its medial borders near the sternum and the upper breast, whereas the lower and outer portions of the breast are more mobile. The technologist takes advantage of the mobile lower outer breast to obtain as much breast tissue on the mammogram as possible. The two views obtained for screening mammography are the craniocaudal (CC) and mediolateral oblique (MLO) projections. The names for the mammographic views and abbreviations are based on the ACR Breast Imaging Reporting and Data System (ACR BI-RADS®), a lexicon system developed by experts for standard mammographic terminology. The first word in the mammographic view indicates the location of the x-ray tube, and the second word indicates the location of the image receptor. Thus, a CC view would be taken with the x-ray tube pointing at the breast from the head (cranial) down through the breast to the image receptor in a more caudal position.

To pass ACR accreditation clinical image review, the MLO mammogram must show most of the breast tissue in one projection, with portions of the upper inner and lower inner quadrants partially excluded (Fig. 1-4). Clinical evaluation of the MLO view should show fat posterior to the fibroglandular tissue and a large portion of the pectoralis muscle, which should be concave and extend inferior to the posterior nipple line (PNL). The PNL describes an imaginary line drawn from the nipple to the pectoralis muscle or film edge and perpendicular to the pectoralis muscle. The PNL should intersect the pectoralis muscle in the MLO view in more than 80% of women. Although the technologist tries to avoid producing skin folds on the film when possible, they are seen occasionally but do not usually cause problems for the radiologist reading the film. The MLO view should show adequate compression, exposure, contrast, and an open inframammary fold, in which both the lower portion of the breast and a portion of the upper abdominal wall should be seen.

Figure 1-4 Positioning for a normal mediolateral oblique (MLO) mammogram. By convention, the view type and side (R, L) labels are placed near the axilla. On a properly positioned MLO view, the inferior aspect of the pectoralis muscle should extend down to the posterior nipple line, an imaginary line drawn from the nipple back to and perpendicular to the pectoral muscle (double arrow). The anterior margin of the pectoralis muscle should be convex in a properly positioned MLO view. Ideally, the image shows fat posterior to the glandular tissue (star). The open inframammary fold (arrow) and abdomen are displayed with the breast pulled up and away from the chest.

To pass ACR accreditation clinical image review, the CC view should include the medial posterior portions of the breast without sacrificing the outer portions (Figs. 1-5 and 1-6). With proper positioning technique, the technologist should be able to include the medial portion of the breast without rotating the patient medially by lifting the lower medial breast tissue onto the image receptor. The pectoralis muscle should be seen when possible on the CC view. On the CC view, the PNL extends from the nipple to the pectoralis muscle or the edge of the film, whichever comes first, perpendicular to the pectoralis muscle or film edge. For a given breast, the length of the PNL on the CC view should be within 1 cm of its length on the MLO view.

Figure 1-5 Normal craniocaudal (CC) mammogram. The posterior nipple line (PNL) on the CC view is the distance between the nipple and the posterior aspect of the image. The PNL on the CC view (double arrow) should be within 1 cm of the PNL on the mediolateral oblique (MLO) view. The goal is to include posterior medial tissue (excluded on the MLO view) (arrow) and as much retroglandular fat (star) as possible.

Figure 1-6 Improper positioning. A, Inadequate pectoralis muscle and sagging breast tissue on this full-field digital mediolateral oblique view show that the posterior nipple line (PNL) requirements are not met, and the craniocaudal (CC) view is rotated laterally (B). Note the calcifying fibroadenoma on the left. C, In a second patient with a fatty breast, the pectoralis muscle is concave but just barely meets PNL requirements. D, The CC views are adequate.

Clinical images are evaluated on positioning, compression, contrast, proper exposure, random noise (radiographic mottle or quantum mottle produced by varying numbers of x-rays contributing to the image in different locations, even with a uniform object), sharpness, and artifacts (or structured noise). Imaging on a phantom is helpful in evaluating most of these factors, except for positioning and compression (Fig. 1-7). Adequate exposure (to achieve adequate film OD) and adequate contrast (OD difference) are important to ensure detection of subtle abnormalities (Fig. 1-8). Artifacts seen on clinical images include processing artifacts (roller marks, wet pressure marks, guide shoe marks), white specklike artifacts from dust or lint between the fluorescent screen and film emulsion, grid lines from incomplete grid motion, motion artifacts from patient movement (made more likely by longer exposure times), skin folds from positioning, tree static caused by static electricity from low humidity in the dark room, or film handling artifacts (fingerprints, crimp marks, or pressure marks) (Figs. 1-9 to 1-12).

Figure 1-7 Phantom image. Fibers, speck groups, and masses in graduated sizes embedded in a 4.5-cm thick phantom are used to evaluate the mammography system; phantom images are obtained at least weekly and after calibration or servicing of equipment. Minimum score: four fibers, three speck groups, and three masses.

Figure 1-8 A, Underpenetration and an inadequately exposed and compressed breast produce a light film and poor separation of breast tissue; even though the pectoralis muscle is adequately included, skin folds are apparent in the lower portion of the image. B,

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